When I was a kid, there were only small square and round bales of hay. These were easy to store and handle. They needed to be stored indoors to avoid spoilage and could be fed in small quantities to avoid waste. But in the last twenty years large round bales that weigh from 500 to 700 lbs have become very common. The benefit of round bales is that they can be stored outside, although a percentage of the bale is wasted by weathering, and they can be left in a horse's pasture or paddock for longer-term feeding. The cost per pound for a round bale is often less than small bales that require more labor to cut, bale, stack and store and require better protection. For these reasons, large rounds have become very popular for feeding horses outdoors.
Purchasing Round Bales
In some places, it may be easier to find larger round bales than small squares because they're favored by cattle farmers. One of the difficulties with purchasing round bales is that it's difficult to see inside the bale to assess quality. If the hay is baled dry, and the field looks weed free, with the desired proportions of legumes and grasses, all should be well.
But just looking at the outside of a bale may not tell the whole story. It's important to check the hay inside the bale as well. It is possible for portions of the bale to by mouldy or soiled. You'll need to ask to open a bale up to check thoroughly. A bale probe and thermometer are useful to check the moisture levels and the temperature. I once had a hay dealer drop off bales that very quickly started to smell musty. On feeling a few inches into the bale heat was evident. By the time the dealer came to replace the bales the hay was turning sticky and white. If this happens, most farmers and dealers will replace bad bales if you've done everything to store them properly.
Large squares are also available. Because moving them is impossible without a tractor, and they need to be stored more carefully than rounds (they don't shed water like round bales) they're less popular with horse owners who don't have the proper storage and equipment.
Storing Round Bales
One of the benefits of using large round bales is that they can be stored outdoors without protection. But doing this means a large portion of the hay may be weather damaged. Bales that lean on each other may mold where they touch. Calculate in the amount of hay the horses can waste simply by ripping the bales apart, and you've lost a good portion of the hay you've paid for. Ideally, bales should be kept in a leak-free building. But if you don't have a building, tarps held away from the bales with something like old tires or skids so condensation doesn't form is an alternative. This will also discourage large wildlife from helping themselves to your store.
The hay should be stored on rails or skids to prevent the bale from absorbing moisture from the ground. You may have the bales stacked, but I know from experience that without a tractor, getting bales from the top of the stack takes a lot of effort, especially if the bales ice together in the winter.
Feeding Round Bales
Two or three people can roll a round bale without too much trouble. It's messy work, and certainly easier with a tractor. A round bale feeder will prevent waste, and slow feeding net reduces waste dramatically. You can order slow feed netting, or buy hockey or sport netting. Just be sure the mesh is small enough that a horse can't put a hoof or head through the holes and do check for rips frequently. Some horses don't do well on round bales. Some, like my own will spend all day with their head in the bale and get too fat. Some horses will get bullied away from the bale and have a problem getting enough to eat. Slow feed netting will slow down the greedy eaters, but hard keepers may need a little extra hay or concentrates in a place they can relax and eat.
Horses with health issues like COPD will need to be monitored carefully. Even though the bale is not really dusty, just eating with their heads burrowed in the bale might trigger problems. Sand colic is also a concern as horses pick up dropped hay from the ground. Round bales are convenient, but they're not the answer for every situation and every horse.